VACCINES AND SERA 275 



horse begins not with pure toxin but with a mixture of toxin + 

 Gram's iodine or of toxin + antitoxin. Every horse has its 

 own susceptibility, and the treatment requires a good deal of 

 skill. It is impossible to foretell the supply of antitoxin from 

 any particular horse. 



The serum treated aseptically is put into little bottles and 

 "tyndallised," i.e., sterilised by heating to a low temperature 

 on several successive days. It is stored in a dark, cool place, 

 and under these conditions it only loses about one-tenth of its 

 activity, so long as it remains clear, even after several years. 



Sera possessing great activity per unit volume have been 

 sought for by selecting those horses which are active pro- 

 ducers, or by concentrating or "refining" the serum by 

 various procedures, freezing, precipitation, &c. The best 

 method is still to take an active serum, to keep it pure and 

 preserve it carefully without further treatment. 



The titration of the serum is indispensable : only serum of 

 known activity is issued for serum therapy. Hitherto, titration 

 has only been possible by resorting to experimental animals, 

 guinea-pigs. The laboratories of all countries have agreed 

 to adopt the method proposed by Ehrlich. The serum is 

 standardized by mixing with a toxin which has itself been 

 estimated against a standard antitoxin. The activity of the 

 serum is expressed in antitoxic units which, like all units of 

 measure, are conventional. The standard antitoxin is preserved 

 with precautions similar to those with which are guarded our 

 standard weights and measures. 



But in spite of all the precision introduced, we are still 

 dealing here with a biological, not a chemical process. 

 Experience has made clear three points on which too much 

 stress cannot be laid, (i) The serum ought to be inoculated 

 as soon as possible after the appearance of the disease. A 

 little serum given early is worth more than a great deal of 

 serum given too late. Similar guinea-pigs receiving the same 

 dose of toxin may be cured by a small dose at the sixth 

 hour, whereas at the eighth hour even a large dose fails. In 

 hospital, unfortunately, the children are usually brought too 



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